How big is space? What we can measure – and what we cannot

Our cosmic horizon is not a wall

There are two different answers to the question of how big space is. We can calculate the extent of the observable universe; for the universe as a whole, we have no reliably established size. Observable means that signals from this region could, in principle, have reached us since the early stages of cosmic evolution. More distant regions may well exist without their light having reached us yet. This horizon is therefore not a physical boundary. Someone at a distant location would have a different observable patch. Earth does not occupy a privileged central position in the cosmos as a whole. It is merely at the centre of the sphere that we can survey from our own location, much as every observer has their own horizon.

Why 13.8 billion years does not mean 13.8 billion light-years

According to the standard cosmological model, the universe is around 13.8 billion years old. Yet the present-day distance to the furthest regions that are observable in principle is approximately 46 billion light-years. The reason is expansion: while the light was travelling, the distances between widely separated regions that are not gravitationally bound increased. A long light travel time and a present-day distance are therefore different quantities. Doubling the approximate radius gives the frequently quoted diameter of 93 billion light-years. This does not require anything to travel locally through space faster than light. Rather, the cosmological change in distance describes the evolution of the geometry of space. This distinction becomes particularly important when distances spanning billions of light-years are quoted and compared.

Is the entire universe infinite?

Whether the cosmos as a whole is finite or infinite remains an open question. Measurements of its large-scale geometry show that the observable patch is very close to being spatially flat. However, infinity does not automatically follow from this. A very large, spatially curved universe could also appear almost flat within our limited patch. Global topology plays a role too: geometry and the way space is connected are not the same question. Any sound statement of size must therefore always specify which distance and which region it refers to. The robust central conclusion is this: our observable cosmos is immense and can be described quantitatively, but its observational limit does not tell us its total size. It is precisely this limit that makes cosmology interesting, because it clearly separates measurable knowledge from possibilities that remain beyond our direct scrutiny.

Keep exploring

• ALMA: How radio telescopes see the cold nurseries of stars
• Dark energy: what accelerating expansion really means
• Dark matter: why astronomers infer invisible mass